This invention relates to a photo-detector.
Many technologies used for communications and information processing require light-detection elements. In some applications, the sensitivity of the detector is of paramount importance. For example in quantum cryptography for secure communications, detection of single photons is required as part of the cryptographic process. Currently, photomultipliers and avalanche photodiodes are used for detection in such systems but have intrinsic disadvantages. Photomultipliers are mechanically delicate, bulky, use high voltages and are prone to overload problems. They have a low quantum efficiency. Avalanche photodiodes are noisy, use high gain voltages and have high dark counts.
A photodetector based on a single electron transistor has been proposed in xe2x80x9cVery low noise photodetector based on the single electron transistorxe2x80x9d A. N. Cleland et al Appl. Phys. Lett. 61(23) Dec. 7, 1992 pp2820-2822. The device operated with low noise levels but a number of problems remain outstanding. Spurious currents can arise produced by charge carrier sources other than incident photons, which degrade the output of the device. Also, its sensitivity is temperature dependent. In practice, the experimental device needs to be cooled in a dilution refrigerator, which renders it bulky and limits its practicality as a photodetector.
The present invention seeks to provide an improved, photo-detector capable of detecting individual photons.
According to a first aspect of the invention there is provided photo-detector comprising a photo-absorptive region to absorb incident photons and produce charge carriers of opposite polarity in response thereto, first and second electrometers responsive to individual ones of the charge carriers of opposite polarity respectively to provide corresponding electrical outputs, and a comparator responsive to the outputs of the electrometers to provide an electrical detector output corresponding to the incident photons.
By using first and second electrometers, their outputs can be combined to increase sensitivity of the photodetector according to the invention to incident photons and spurious currents produced in the electrometers by other sources can be arranged to cancel one another to reduce spurious outputs from the device.
In another aspect the invention provides a photo-detector comprising a photo-absorptive region to absorb incident photons and produce charge carriers in response thereto, an electrometer responsive to individual ones of the charge carriers to provide a corresponding electrical output, and an electrically driven cooling device to cool the electrometer.
The cooling device may comprise a Peltier effect device.
In a further aspect the invention provides a photo-detector comprising a photo-absorptive region to absorb incident photons and produce charge carriers in response thereto, and an electrometer responsive to individual ones of the charge carriers to provide a corresponding electrical output and a substrate that has a main body portion and a cantilever extending from the main body portion, said photo-absorptive region and said electrometer being disposed in the cantilever. The cantilevered structure facilitates arrays of the devices to be formed on a substrate.